Stimulation of the T cell antigen receptor (TCR) leads to the tyrosine phosphorylation of phospholipase C (PLC)gamma1, and the weight of current evidence suggests that activation of a protein tyrosine kinase (PTK) is required for TCR-mediated polyphosphoinositide turnover. To identify molecules involved in the coupling of the TCR to the activation of PLCgamma1, I have determined whether stimulation of the TCR induces the tyrosine phosphorylation of molecules that bind the src-homology 2 (SH2) domains of PLCgamma1. Using TrpE fusion proteins containing the SH2 domains of PLCgamma1 as an affinity ligand, I have identified tyrosine- phosphorylated proteins of Mr 23, 39, 40, and 63 kD that, following TCR stimulation, precipitate with the PLCgamma1 SH2 fusion protein. Based on these initial studies, I propose the following hypothesis: stimulation of the TCR leads to the tyrosine phosphorylation of regulatory molecules that bind to the SH2 domains of PLCgamma1 and that are required for the coupling of TCR stimulation to the activation of PLCgamma1. The following studies will be initiated during phase I of the proposal. I will determine whether the precipitation of these proteins with PLCgamma1 SH2 is specific for TCR- mediated signaling. I will determine whether the 63 kD molecule is one of the PTKs implicated in TCR signaling (Fyn or Lck) and whether the 23 kD protein is the TCR zeta chain. I will purify sufficient pp39/pp40 to obtain protein sequence for the cloning of the gene(s) that encodes these proteins. I will determine whether expression of the PLCgamma1 SH2 domain within intact Jurkat cells can block TCR-mediated activation of endogenous PLC.